Hello Miss,
I'd like to tell you that you are in a real messy now and it's a big dilemna to choose between your friend and the boy you love. Whatsoever, you will hurt one of them even if you don't want to.
Of course, I won't judge you concerning the fact that the boy you love now was your best friend's boyfriend. But if you first have think about the trouble it will lead probably you will avoid such situation.
Despite all, if I were at your place I would probably tell my friend the truth
otherwise you will always feel bad. It will help you if you try to explain your situation and if you have a lack of courage, you and your boyfriend could tell her the truth sincerly.
Honestly, it's so difficult because perhaps you will lost her friendship. Now, the best you can do is to be honest.
I hope my advice will help you to take the best decision. Good luck and my
best regards ........................(tu mets ton prénom)